The present invention relates to a control for controlling a reduction ratio in a continuously variable transmission.
In controlling a reduction ratio in a continuously variable transmission of a vehicle having an engine, a vehicle speed, an engine speed and a throttle opening are sensed or detected and a desired reduction ratio is so determined based on the sensed results as to operate the engine in a predetermined desired operating state thereof. However, if the reduction ratio in the continuously variable transmission is controlled in the same shift schedule or pattern; a problem is posed when the engine operation is irregular during engine start-up and the subsequent warming up period where the engine temperature is low. The problem is in that the engine cannot produce power high enough to meet the demand, the engine vibrates unplesantly and the engine stalls.
One proposal to alleviate the above problem is described in a co-pending U.S. Patent Application Ser. No. 486,331 filed Apr. 19, 1983 which has been commonly assigned herewith (corresponding to European Patent Application No. 83103773.4 filed Apr. 19, 1983). According to this known control idea, when the engine coolant temperature is lower than a predetermined value, the sensed vehicle speed or engine speed or throttle operating condition is corrected so as to increase the desired reduction ratio toward the larger reduction ratio side in accordance with the degree to which the engine coolant temperature drops below the predetermined value, thus increasing the engine speed. This control strategy, however, is not satisfactory in solving the above mentioned problem because there still remains a possibility that the transmission is set to a minimum reduction ratio which is an achievable limit on account of the construction of the transmission when the vehicle plunges into a certain operating condition. Therefore, even with this known control, the above mentioned problem is not solved although it is somewhat alleviated.